Storage phosphor based radiographic imaging systems (computer radiography) are currently in widespread use. These systems use screens containing phosphor material that store a portion of the incident ionizing radiation as latent storage sites. These sites are subsequently stimulated to release electromagnetic radiation (the stimulated emission), typically in the 350 to 450 nanometer range, in proportion to the amount of ionizing radiation that was absorbed by the phosphor material. The typical readout method used in these systems is the so-called flying-spot scanning method. A focused laser beam, typically in the 600 to 700 nanometer range, is raster scanned over the surface of the screen to stimulate the storage sites. Synchronously, the stimulated emission is collected, detected, and digitized. The pixel size of the image is determined by the raster rate and digitization rate. After readout, the screens are flooded with erasing light to remove any remaining storage sites so the screen can be reused.
An alternative configuration described in U.S. Pat. No. 6,373,074, issued Apr. 16, 2002, inventors Mueller et al., and U.S. patent application Publication 2002/0008212A1, published Jan. 24, 2002, inventors Arakawa et al., is one where a line of stimulating electromagnetic radiation is used, and the stimulated emission is re-imaged onto a linear segmented detector such as a photodiode array or a charge-coupled device (CCD). For this line stimulation, the pixel size is determined by the digitization rate in one direction, and by the optical imaging and detection system in the other direction.
One of the challenges for any configuration of stimulation and detection is collecting a large fraction of the stimulated emission so as to obtain high image quality. The stimulated emission is emitted in a broad angular range. For most systems, the emission is close to being Lambertian (a cos(θ) fall off in intensity with angle of emission). For the raster-scanned systems, the typical collection systems have a large acceptance angle for the stimulated emission, and are highly reflective and shaped so that the emission is directed to a fairly large area detector, such as, a photomultiplier tube. For some systems, the collector is a light-pipe, i.e., a plastic conduit that uses total internal reflection to guide the stimulated emission to the detector. Given that the typical stimulated emission wavelength range is 350 to 450 nanometers, the plastic must have a high transmittance for ultraviolet and blue electromagnetic radiation. For the imaged line-stimulation systems, the collection optics used must have a very low f-number to collect a large fraction of the emission. This places constraints on the depth of field of such an imaging system. Also, such low f-number optics can be more expensive than higher f-number optics. If the range of emission angles could be narrowed, the collection optics could be greatly simplified, thus saving space and cost. One such method of altering the emission angle range is disclosed in U.S. Pat. No. 6,507,032, issued Jan. 14, 2003, inventors Hell et al., in which microlenses are formed on the surface of the screen in an attempt to narrow the range of emission angles. This technique can only slightly narrow the emission cone, and adds manufacturing cost to each screen. It also does not alter the wavelength of the emission as discussed in the next paragraph.
Another challenge is to detect the stimulated emission with very high quantum efficiency (QE). The typical wavelength of the emission is 350 to 450 nanometers. For the raster-scanned systems, the detector is typically a photomultiplier tube (PMT). The QE of a typical PMT has a value around 25% at 400 nanometers for a bi-alkali photocathode. For the re-imaged systems, typical CCD detectors have a QE at 400 nm that is typically 50% or lower. If the wavelength of the emission could be shifted towards longer wavelength, then CCD and other semiconductor detectors will detect the emission with higher QE.